c-di-GMP signalling and the regulation of developmental transitions in streptomycetes

Allosteric regulation by c-di-AMP modulates a complete N-acetylglucosamine signaling cascade in Saccharopolyspora erythraea

c-di-AMP is an essential and widespread nucleotide second messenger in bacterial signaling. For most c-di-AMP synthesizing organisms, c-di-AMP homeostasis and the molecular mechanisms pertaining to its signal transduction are of great concern. Here we show that c-di-AMP binds the N-acetylglucosamine (GlcNAc)-sensing regulator DasR, indicating a direct link between c-di-AMP and GlcNAc signaling. Beyond its foundational role in cell-surface structure, GlcNAc is attractive as a major nutrient and messenger molecule regulating multiple cellular processes from bacteria to humans. We show that increased c-di-AMP levels allosterically activate DasR as a master repressor of GlcNAc utilization, causing the shutdown of the DasR-mediated GlcNAc signaling cascade and leading to a consistent enhancement in the developmental transition and antibiotic production in Saccharopolyspora erythraea. The expression of disA, encoding diadenylate cyclase, is directly repressed by the regulator DasR in response to GlcNAc signaling, thus forming a self-sustaining transcriptional feedback loop for c-di-AMP synthesis. These findings shed light on the allosteric regulation by c-di-AMP, which appears to play a prominent role in global signal integration and c-di-AMP homeostasis in bacteria and is likely widespread in streptomycetes that produce c-di-AMP.

CslA and GlxA from Streptomyces lividans form a functional cellulose synthase complex

Filamentous growth of streptomycetes coincides with the synthesis and deposition of an uncharacterized protective glucan at hyphal tips. Synthesis of this glucan depends on the integral membrane protein CslA and the radical copper oxidase GlxA, which are part of a presumably large multiprotein complex operating at growing tips. Here, we show that CslA and GlxA interact by forming a protein complex that is sufficient to synthesize cellulose in vitro. Mass spectrometry analysis revealed that the purified complex produces cellulose chains with a degree of polymerization of at least 80 residues. Truncation analyses demonstrated that the removal of a significant extracellular segment of GlxA had no impact on complex formation, but significantly diminished activity of CslA. Altogether, our work demonstrates that CslA and GlxA form the active core of the cellulose synthase complex and provide molecular insights into a unique cellulose biosynthesis system that is conserved in streptomycetes.

IMPORTANCE

Cellulose stands out as the most abundant polysaccharide on Earth. While the synthesis of this polysaccharide has been extensively studied in plants and Gram-negative bacteria, the mechanisms in Gram-positive bacteria have remained largely unknown. Our research unveils a novel cellulose synthase complex formed by the interaction between the cellulose synthase-like protein CslA and the radical copper oxidase GlxA from Streptomyces lividans, a soil-dwelling Gram-positive bacterium. This discovery provides molecular insights into the distinctive cellulose biosynthesis machinery. Beyond expanding our understanding of cellulose biosynthesis, this study also opens avenues for exploring biotechnological applications and ecological roles of cellulose in Gram-positive bacteria, thereby contributing to the broader field of microbial cellulose biosynthesis and biofilm research.

Building a highly efficient Streptomyces super-chassis for secondary metabolite production by reprogramming naturally-evolved multifaceted shifts

Streptomyces has an extensive array of bioactive secondary metabolites (SMs). Nevertheless, devising a framework for the heterologous production of these SMs remains challenging. We here reprogrammed a versatile plug-and-play Streptomyces super-chassis and established a universal pipeline for production of diverse SMs via understanding of the inherent pleiotropic effects of ethanol shock on jadomycin production in Streptomyces venezuelae. We initially identified and characterized a set of multiplex targets (afsQ1, bldD, bldA, and miaA) that contribute to SM (jadomycin) production when subjected to ethanol shock. Subsequently, we developed an ethanol-induced orthogonal amplification system (EOAS), enabling dynamic and precise control over targets. Ultimately, we integrated these multiplex targets into functional units governed by the EOAS, generating a universal and plug-and-play Streptomyces super-chassis. In addition to achieving the unprecedented titer and yield of jadomycin B, we also evidenced the potential of this super-chassis for production of diverse heterologous SMs, including antibiotic oxytetracycline, anticancer drug doxorubicins, agricultural herbicide thaxtomin A, and plant growth regulator guvermectin, all with the yields of >10 mg/g glucose in a simple mineral medium. Given that the production of SMs all required complexed medium and the cognate yields were usually much lower, our achievement of using a universal super-chassis and engineering pipeline in a simple mineral medium is promising for convenient heterologous production of SMs.

Interactions between plant-beneficial microorganisms in a consortium: Streptomyces microflavus and Trichoderma harzianum

The construction of microbial consortia is challenging due to many variables to be controlled, including the cross-compatibility of the selected strains and their additive or synergistic effects on plants. In this work, we investigated the interactions in vitro, in planta, and at the molecular level of two elite biological control agents (BCAs), that is Streptomyces microflavus strain AtB-42 and Trichoderma harzianum strain M10, to understand their attitude to cooperate in a consortium. In vitro, we observed a strong cross-antagonism between AtB-42 and M10 in agar plates due to diffusible metabolites and volatile organic compounds. In liquid co-cultures, M10 hindered the growth of AtB-42 very likely because of secondary metabolites and strong competition for the nutrients. The interaction in the co-culture induced extensive transcriptional reprogramming in both strains, especially in the pathways related to ribosomes, protein synthesis, and oxidoreductase activity, suggesting that each strain recognized the counterpart and activated its defence responses. The metabolome of both strains was also significantly affected. In contrast, in the soil, M10 growth was partially contrasted by AtB-42. The roots of tomato seedlings inoculated with the consortium appeared smaller than the control and single-strain-inoculated plants, indicating that plants diverted some energy from the development to defence activation, as evidenced by the leaf transcriptome. The consortium induced a stronger transcriptional change compared to the single inoculants, as demonstrated by a higher number of differentially expressed genes. Although the cross-antagonism observed in vitro, the two strains exerted a synergistic effect on tomato seedlings by inducing resistance responses stronger than the single inoculants. Our observations pose a question on the usefulness of the sole in vitro assays for selecting BCAs to construct a consortium. In vivo experiments should be preferred, and transcriptomics may greatly help to elucidate the activity of the BCAs beyond the phenotypic effects on the plant.

The common origin and degenerative evolution of flagella in Actinobacteria

IMPORTANCE

Flagellar motility plays an important role in the environmental adaptation of bacteria and is found in more than 50% of known bacterial species. However, this important characteristic is sparsely distributed within members of the phylum Actinobacteria, which constitutes one of the largest bacterial groups. It is unclear why this important fitness organelle is absent in most actinobacterial species and the origin of flagellar genes in other species. Here, we present detailed analyses of the evolution of flagellar genes in Actinobacteria, in conjunction with the ecological distribution and cell biological features of major actinobacterial lineages, and the co-evolution of signal transduction systems. The results presented in addition to clarifying the puzzle of sporadic distribution of flagellar motility in Actinobacteria, also provide important insights into the evolution of major lineages within this phylum.

Structures of the DarR transcription regulator reveal unique modes of second messenger and DNA binding

The mycobacterial repressor, DarR, a TetR family regulator (TFR), was the first transcription regulator shown to bind c-di-AMP. However, the molecular basis for this interaction and the mechanism involved in DNA binding by DarR remain unknown. Here we describe DarR-c-di-AMP and DarR-DNA structures and complementary biochemical assays. The DarR-c-di-AMP structure reveals a unique effector binding site for a TFR, located between DarR dimer subunits. Strikingly, we show this motif also binds cAMP. The location of the adenine nucleotide binding site between subunits suggests this interaction may facilitate dimerization and hence DNA binding. Indeed, biochemical assays show cAMP enhances DarR DNA binding. Finally, DarR-DNA structures reveal a distinct TFR DNA-binding mechanism involving two interacting dimers on the DNA. Thus, the combined data unveil a newly described second messenger binding motif and DNA binding mode for this important family of regulators.

Phylogenetic Analysis and Characterization of Diguanylate Cyclase and Phosphodiesterase in Planktonic Filamentous Cyanobacterium Arthrospira sp

Cyclic di-GMP (c-di-GMP) is a second messenger of intracellular communication in bacterial species, which widely modulates diverse cellular processes. However, little is known about the c-di-GMP network in filamentous multicellular cyanobacteria. In this study, we preliminarily investigated the c-di-GMP turnover proteins in Arthrospira based on published protein data. Bioinformatics results indicate the presence of at least 149 potential turnover proteins in five Arthrospira subspecies. Some proteins are highly conserved in all tested Arthrospira, whereas others are specifically found only in certain subspecies. To further validate the protein catalytic activity, we constructed a riboswitch-based c-di-GMP expression assay system in Escherichia coli and confirmed that a GGDEF domain protein, Adc11, exhibits potential diguanylate cyclase activity. Moreover, we also evaluated a protein with a conserved HD-GYP domain, Ahd1, the expression of which significantly improved the swimming ability of E. coli. Enzyme-linked immunosorbent assay also showed that overexpression of Ahd1 reduced the intracellular concentration of c-di-GMP, which is presumed to exhibit phosphodiesterase activity. Notably, meta-analyses of transcriptomes suggest that Adc11 and Ahd1 are invariable. Overall, this work confirms the possible existence of a functional c-di-GMP network in Arthrospira, which will provide support for the revelation of the biological function of the c-di-GMP system in Arthrospira.

Effects of a Pirin-like protein on strain growth and spinosad biosynthesis in Saccharopolyspora spinosa

Pirin family proteins perform a variety of biological functions and widely exist in all living organisms. A few studies have shown that Pirin family proteins may be involved in the biosynthesis of antibiotics in actinomycetes. However, the function of Pirin-like proteins in S. spinosa is still unclear. In this study, the inactivation of the sspirin gene led to serious growth defects and the accumulation of H2O2. Surprisingly, the overexpression and knockout of sspirin slightly accelerated the consumption and utilization of glucose, weakened the TCA cycle, delayed sporulation, and enhanced sporulation in the later stage. In addition, the overexpression of sspirin can enhance the β-oxidation pathway and increase the yield of spinosad by 0.88 times, while the inactivation of sspirin hardly produced spinosad. After adding MnCl2, the spinosad yield of the sspirin overexpression strain was further increased to 2.5 times that of the wild-type strain. This study preliminarily revealed the effects of Pirin-like proteins on the growth development and metabolism of S. spinosa and further expanded knowledge of Pirin-like proteins in actinomycetes. KEY POINTS: • Overexpression of the sspirin gene possibly triggers carbon catabolite repression (CCR) • Overexpression of the sspirin gene can promote the synthesis of spinosad • Knockout of the sspirin gene leads to serious growth and spinosad production defects.

Genus Streptomyces: Recent advances for biotechnological purposes

Actinomycetes are a distinct group of filamentous bacteria. The Streptomyces genus within this group has been extensively studied over the years, with substantial contributions to society and science. This genus is known for its antimicrobial production, as well as antitumor, biopesticide, and immunomodulatory properties. Therefore, the extraordinary plasticity of the Streptomyces genus has inspired new research techniques. The newest way of exploring Streptomyces has comprised the discovery of new natural metabolites and the application of emerging tools such as CRISPR technology in drug discovery. In this narrative review, we explore relevant published literature concerning the ongoing novelties of the Streptomyces genus.

The Best of Both Worlds-Streptomyces coelicolor and Streptomyces venezuelae as Model Species for Studying Antibiotic Production and Bacterial Multicellular Development

Streptomyces bacteria have been studied for more than 80 years thanks to their ability to produce an incredible array of antibiotics and other specialized metabolites and their unusual fungal-like development. Their antibiotic production capabilities have ensured continual interest from both academic and industrial sectors, while their developmental life cycle has provided investigators with unique opportunities to address fundamental questions relating to bacterial multicellular growth. Much of our understanding of the biology and metabolism of these fascinating bacteria, and many of the tools we use to manipulate these organisms, have stemmed from investigations using the model species Streptomyces coelicolor and Streptomyces venezuelae. Here, we explore the pioneering work in S. coelicolor that established foundational genetic principles relating to specialized metabolism and development, alongside the genomic and cell biology developments that led to the emergence of S. venezuelae as a new model system. We highlight key discoveries that have stemmed from studies of these two systems and discuss opportunities for future investigations that leverage the power and understanding provided by S. coelicolor and S. venezuelae.

Evidence of a role for CutRS and actinorhodin in the secretion stress response in Streptomyces coelicolor M145

CutRS was the first two-component system to be identified in Streptomyces species and is highly conserved in this genus. It was reported >25 years ago that deletion of cutRS increases the production of the antibiotic actinorhodin in Streptomyces coelicolor. However, despite this early work, the function of CutRS has remained enigmatic until now. Here we show that deletion of cutRS upregulates the production of the actinorhodin biosynthetic enzymes up to 300-fold, explaining the increase in actinorhodin production. However, while ChIP-seq identified 85 CutR binding sites in S. coelicolor none of these are in the actinorhodin biosynthetic gene cluster, meaning the effect is indirect. The directly regulated CutR targets identified in this study are implicated in extracellular protein folding, including two of the four highly conserved HtrA-family foldases: HtrA3 and HtrB, and a putative VKOR enzyme, which is predicted to recycle DsbA following its catalysis of disulphide bond formation in secreted proteins. Thus, we tentatively propose a role for CutRS in sensing and responding to protein misfolding outside the cell. Since actinorhodin can oxidise cysteine residues and induce disulphide bond formation in proteins, its over production in the ∆cutRS mutant may be a response to protein misfolding on the extracellular face of the membrane.

Global Effects of the Developmental Regulator BldB in Streptomyces venezuelae

In Streptomyces, the Bld (Bald) regulators control formation of the reproductive aerial hyphae. The functions of some of these regulators have been well characterized, but BldB has remained enigmatic. In addition to the bldB gene itself, Streptomyces venezuelae has 10 paralogs of bldB that sit next to paralogs of whiJ and abaA. Transcriptome sequencing (RNA-seq) revealed that loss of BldB function causes the dramatic transcriptional upregulation of the abaA paralogs and a novel inhibitor of sporulation, iosA, and that cooverexpression of just two of these genes, iosA and abaA6, was sufficient to recapitulate the bldB mutant phenotype. Further RNA-seq analysis showed that the transcription factor WhiJ9 is required for the activation of iosA seen in the bldB mutant, and biochemical studies showed that WhiJ9 mediates the activation of iosA expression by binding to direct repeats in the iosA-whiJ9 intergenic region. BldB and BldB9 hetero-oligomerize, providing a potential link between BldB and the iosA-whiJ9-bldB9 locus. This work greatly expands our overall understanding of the global effects of the BldB developmental regulator. IMPORTANCE To reproduce and disperse, the filamentous bacterium Streptomyces develops specialized reproductive structures called aerial hyphae. The formation of these structures is controlled by the bld (bald) genes, many of which encode transcription factors whose functions have been characterized. An exception is BldB, a protein whose biochemical function is unknown. In this study, we gain insight into the global effects of BldB function by examining the genome-wide transcriptional effects of deleting bldB. We identify a small set of genes that are dramatically upregulated in the absence of BldB. We show that their overexpression causes the bldB phenotype and characterize a transcription factor that mediates the upregulation of one of these target genes. Our results provide new insight into how BldB influences Streptomyces development.

Local signaling enhances output specificity of bacterial c-di-GMP signaling networks

For many years the surprising multiplicity, signal input diversity, and output specificity of c-di-GMP signaling proteins has intrigued researchers studying bacterial second messengers. How can several signaling pathways act in parallel to produce specific outputs despite relying on the same diffusible second messenger maintained at a certain global cellular concentration? Such high specificity and flexibility arise from combining modes of local and global c-di-GMP signaling in complex signaling networks. Local c-di-GMP signaling can be experimentally shown by three criteria being met: (i) highly specific knockout phenotypes for particular c-di-GMP-related enzymes, (ii) actual cellular c-di-GMP levels that remain unchanged by such mutations and/or below the K_d's of the relevant c-di-GMP-binding effectors, and (iii) direct interactions between the signaling proteins involved. Here, we discuss the rationale behind these criteria and present well-studied examples of local c-di-GMP signaling in Escherichia coli and Pseudomonas. Relatively simple systems just colocalize a local source and/or a local sink for c-di-GMP, i.e. a diguanylate cyclase (DGC) and/or a specific phosphodiesterase (PDE), respectively, with a c-di-GMP-binding effector/target system. More complex systems also make use of regulatory protein interactions, e.g. when a "trigger PDE" responds to locally provided c-di-GMP, and thereby serves as a c-di-GMP-sensing effector that directly controls a target's activity, or when a c-di-GMP-binding effector recruits and directly activates its own "private" DGC. Finally, we provide an outlook into how cells can combine local and global signaling modes of c-di-GMP and possibly integrate those into other signaling nucleotides networks.

Recent advances and perspectives in nucleotide second messenger signaling in bacteria

Nucleotide second messengers act as intracellular 'secondary' signals that represent environmental or cellular cues, i.e. the 'primary' signals. As such, they are linking sensory input with regulatory output in all living cells. The amazing physiological versatility, the mechanistic diversity of second messenger synthesis, degradation, and action as well as the high level of integration of second messenger pathways and networks in prokaryotes has only recently become apparent. In these networks, specific second messengers play conserved general roles. Thus, (p)ppGpp coordinates growth and survival in response to nutrient availability and various stresses, while c-di-GMP is the nucleotide signaling molecule to orchestrate bacterial adhesion and multicellularity. c-di-AMP links osmotic balance and metabolism and that it does so even in Archaea may suggest a very early evolutionary origin of second messenger signaling. Many of the enzymes that make or break second messengers show complex sensory domain architectures, which allow multisignal integration. The multiplicity of c-di-GMP-related enzymes in many species has led to the discovery that bacterial cells are even able to use the same freely diffusible second messenger in local signaling pathways that can act in parallel without cross-talking. On the other hand, signaling pathways operating with different nucleotides can intersect in elaborate signaling networks. Apart from the small number of common signaling nucleotides that bacteria use for controlling their cellular "business," diverse nucleotides were recently found to play very specific roles in phage defense. Furthermore, these systems represent the phylogenetic ancestors of cyclic nucleotide-activated immune signaling in eukaryotes.

Osmotic stress responses and the biology of the second messenger c-di-AMP in Streptomyces

Streptomyces are prolific antibiotic producers that thrive in soil, where they encounter diverse environmental cues, including osmotic challenges caused by rainfall and drought. Despite their enormous value in the biotechnology sector, which often relies on ideal growth conditions, how Streptomyces react and adapt to osmotic stress is heavily understudied. This is likely due to their complex developmental biology and an exceptionally broad number of signal transduction systems. With this review, we provide an overview of Streptomyces' responses to osmotic stress signals and draw attention to open questions in this research area. We discuss putative osmolyte transport systems that are likely involved in ion balance control and osmoadaptation and the role of alternative sigma factors and two-component systems (TCS) in osmoregulation. Finally, we highlight the current view on the role of the second messenger c-di-AMP in cell differentiation and the osmotic stress responses with specific emphasis on the two models, S. coelicolor and S. venezuelae.

Structural basis of dual activation of cell division by the actinobacterial transcription factors WhiA and WhiB

Studies of transcriptional initiation in different bacterial clades reveal diverse molecular mechanisms regulating this first step in gene expression. The WhiA and WhiB factors are both required to express cell division genes in Actinobacteria and are essential in notable pathogens such as Mycobacterium tuberculosis. The WhiA/B regulons and binding sites have been elucidated in Streptomyces venezuelae (Sven), where they coordinate to activate sporulation septation. However, how these factors cooperate at the molecular level is not understood. Here we present cryoelectron microscopy structures of Sven transcriptional regulatory complexes comprising RNA polymerase (RNAP) σA-holoenzyme and WhiA and WhiB, in complex with the WhiA/B target promoter sepX. These structures reveal that WhiB binds to domain 4 of σA (σA4) of the σA-holoenzyme, bridging an interaction with WhiA while making non-specific contacts with the DNA upstream of the -35 core promoter element. The N-terminal homing endonuclease-like domain of WhiA interacts with WhiB, while the WhiA C-terminal domain (WhiA-CTD) makes base-specific contacts with the conserved WhiA GACAC motif. Notably, the structure of the WhiA-CTD and its interactions with the WhiA motif are strikingly similar to those observed between σA4 housekeeping σ-factors and the -35 promoter element, suggesting an evolutionary relationship. Structure-guided mutagenesis designed to disrupt these protein-DNA interactions reduces or abolishes developmental cell division in Sven, confirming their significance. Finally, we compare the architecture of the WhiA/B σA-holoenzyme promoter complex with the unrelated but model CAP Class I and Class II complexes, showing that WhiA/WhiB represent a new mechanism in bacterial transcriptional activation.

Cytoplasmic contractile injection systems mediate cell death in Streptomyces

Contractile injection systems (CIS) are bacteriophage tail-like structures that mediate bacterial cell-cell interactions. While CIS are highly abundant across diverse bacterial phyla, representative gene clusters in Gram-positive organisms remain poorly studied. Here we characterize a CIS in the Gram-positive multicellular model organism Streptomyces coelicolor and show that, in contrast to most other CIS, S. coelicolor CIS (CIS^Sc) mediate cell death in response to stress and impact cellular development. CIS^Sc are expressed in the cytoplasm of vegetative hyphae and are not released into the medium. Our cryo-electron microscopy structure enabled the engineering of non-contractile and fluorescently tagged CIS^Sc assemblies. Cryo-electron tomography showed that CIS^Sc contraction is linked to reduced cellular integrity. Fluorescence light microscopy furthermore revealed that functional CIS^Sc mediate cell death upon encountering different types of stress. The absence of functional CIS^Sc had an impact on hyphal differentiation and secondary metabolite production. Finally, we identified three putative effector proteins, which when absent, phenocopied other CIS^Sc mutants. Our results provide new functional insights into CIS in Gram-positive organisms and a framework for studying novel intracellular roles, including regulated cell death and life-cycle progression in multicellular bacteria.

An overview on the two-component systems of Streptomyces coelicolor

The two-component system (TCS) found in various organisms is a regulatory system, which is involved in the response by the organism to stimuli, thereby regulating the internal behavior of the cell. It is commonly found in prokaryotes and is an important signaling system in bacteria. TCSs are involved in the regulation of physiological and morphological differentiation of the industrially important microbes from the genus Streptomyces, which produce a vast array of bioactive secondary metabolites (SMs). Genetic engineering of TCSs can substantially increase the yield of target SMs, which is valuable for industrial-scale production. Research on TCS has mainly been completed in the model strain Streptomyces coelicolor. In this review, we summarize the recent advances in the functional identification and elucidation of the regulatory mechanisms of various TCSs in S. coelicolor, with a focus on their roles in the biosynthesis of important SMs.

Diversity, mechanisms and beneficial features of phosphate-solubilizing Streptomyces in sustainable agriculture: A review

Currently, the use of phosphate (P) biofertilizers among many bioformulations has attracted a large amount of interest for sustainable agriculture. By acting as growth promoters, members of the Streptomyces genus can positively interact with plants. Several studies have shown the great potential of this bacterial group in supplementing P in a soluble, plant-available form by several mechanisms. Furthermore, some P-solubilizing Streptomyces (PSS) species are known as plant growth-promoting rhizobacteria that are able to promote plant growth through other means, such as increasing the availability of soil nutrients and producing a wide range of antibiotics, phytohormones, bioactive compounds, and secondary metabolites other than antimicrobial compounds. Therefore, the use of PSS with multiple plant growth-promoting activities as an alternative strategy appears to limit the negative impacts of chemical fertilizers in agricultural practices on environmental and human health, and the potential effects of these PSS on enhancing plant fitness and crop yields have been explored. However, compared with studies on the use of other gram-positive bacteria, studies on the use of Streptomyces as P solubilizers are still lacking, and their results are unclear. Although PSS have been reported as potential bioinoculants in both greenhouse and field experiments, no PSS-based biofertilizers have been commercialized to date. In this regard, this review provides an overview mainly of the P solubilization activity of Streptomyces species, including their use as P biofertilizers in competitive agronomic practices and the mechanisms through which they release P by solubilization/mineralization, for both increasing P use efficiency in the soil and plant growth. This review further highlights and discusses the beneficial association of PSS with plants in detail with the latest developments and research to expand the knowledge concerning the use of PSS as P biofertilizers for field applications by exploiting their numerous advantages in improving crop production to meet global food demands.

Global Regulator AdpA_1075 Regulates Morphological Differentiation and Ansamitocin Production in Actinosynnema pretiosum subsp. auranticum

Actinosynnema pretiosum is a well-known producer of maytansinoid antibiotic ansamitocin P-3 (AP-3). Growth of A. pretiosum in submerged culture was characterized by the formation of complex mycelial particles strongly affecting AP-3 production. However, the genetic determinants involved in mycelial morphology are poorly understood in this genus. Herein a continuum of morphological types of a morphologically stable variant was observed during submerged cultures. Expression analysis revealed that the ssgA_6663 and ftsZ_5883 genes are involved in mycelial aggregation and entanglement. Combing morphology observation and morphology engineering, ssgA_6663 was identified to be responsible for the mycelial intertwining during liquid culture. However, down-regulation of ssgA_6663 transcription was caused by inactivation of adpA_1075, gene coding for an AdpA-like protein. Additionally, the overexpression of adpA_1075 led to an 85% increase in AP-3 production. Electrophoretic mobility shift assays (EMSA) revealed that AdpA_1075 may bind the promoter regions of asm28 gene in asm gene cluster as well as the promoter regions of ssgA_6663. These results confirm that adpA_1075 plays a positive role in AP-3 biosynthesis and morphological differentiation.

Comparative and pangenomic analysis of the genus Streptomyces

Streptomycetes are highly metabolically gifted bacteria with the abilities to produce bioproducts that have profound economic and societal importance. These bioproducts are produced by metabolic pathways including those for the biosynthesis of secondary metabolites and catabolism of plant biomass constituents. Advancements in genome sequencing technologies have revealed a wealth of untapped metabolic potential from Streptomyces genomes. Here, we report the largest Streptomyces pangenome generated by using 205 complete genomes. Metabolic potentials of the pangenome and individual genomes were analyzed, revealing degrees of conservation of individual metabolic pathways and strains potentially suitable for metabolic engineering. Of them, Streptomyces bingchenggensis was identified as a potent degrader of plant biomass. Polyketide, non-ribosomal peptide, and gamma-butyrolactone biosynthetic enzymes are primarily strain specific while ectoine and some terpene biosynthetic pathways are highly conserved. A large number of transcription factors associated with secondary metabolism are strain-specific while those controlling basic biological processes are highly conserved. Although the majority of genes involved in morphological development are highly conserved, there are strain-specific varieties which may contribute to fine tuning the timing of cellular differentiation. Overall, these results provide insights into the metabolic potential, regulation and physiology of streptomycetes, which will facilitate further exploitation of these important bacteria.

Allosteric regulation of glycogen breakdown by the second messenger cyclic di-GMP

Streptomyces are our principal source of antibiotics, which they generate concomitant with a complex developmental transition from vegetative hyphae to spores. c-di-GMP acts as a linchpin in this transition by binding and regulating the key developmental regulators, BldD and WhiG. Here we show that c-di-GMP also binds the glycogen-debranching-enzyme, GlgX, uncovering a direct link between c-di-GMP and glycogen metabolism in bacteria. Further, we show c-di-GMP binding is required for GlgX activity. We describe structures of apo and c-di-GMP-bound GlgX and, strikingly, their comparison shows c-di-GMP induces long-range conformational changes, reorganizing the catalytic pocket to an active state. Glycogen is an important glucose storage compound that enables animals to cope with starvation and stress. Our in vivo studies reveal the important biological role of GlgX in Streptomyces glucose availability control. Overall, we identify a function of c-di-GMP in controlling energy storage metabolism in bacteria, which is widespread in Actinobacteria.

Involvement of BldC in the Formation of Physiologically Mature Sporangium in Actinoplanes missouriensis

AmBldD is a global transcriptional regulator that represses the transcription of several genes required for sporangium formation in Actinoplanes missouriensis. Here, we characterized one of the AmBldD regulons: AMIS_1980, encoding an ortholog of BldC, which is a transcriptional regulator involved in the morphological development of Streptomyces. We determined the transcriptional start point of the bldC ortholog by high-resolution S1 nuclease mapping and found an AmBldD box in its 5'-untranslated region. Reverse transcription-quantitative PCR analysis revealed that the transcription of bldC is activated during sporangium formation. A bldC null mutant (ΔbldC) strain formed normally shaped sporangia, but they exhibited defective sporangium dehiscence; under a dehiscence-inducing condition, the number of spores released from the sporangia of the ΔbldC strain was 2 orders of magnitude lower than that from the sporangia of the wild-type strain. RNA sequencing analysis indicated that BldC functions as a transcriptional activator of several developmental genes, including tcrA, which encodes a key transcriptional activator that regulates sporangium formation, sporangium dehiscence, and spore dormancy. Using electrophoretic mobility shift assay (EMSA), we showed that a recombinant BldC protein directly binds to upstream regions of at least 18 genes, the transcription of which is downregulated in the ΔbldC strain. Furthermore, using DNase I footprinting and EMSA, we demonstrated that BldC binds to the direct repeat sequences containing an AT-rich motif. Thus, BldC is a global regulator that activates the transcription of several genes, some of which are likely to be required for sporangium dehiscence. IMPORTANCE BldC is a global transcriptional regulator that acts as a "brake" in the morphological differentiation of Streptomyces. BldC-like proteins are widely distributed throughout eubacteria, but their orthologs have not been studied outside streptomycetes. Here, we revealed that the BldC ortholog in Actinoplanes missouriensis is essential for sporangium dehiscence and that its regulon is different from the BldC regulon in Streptomyces venezuelae, suggesting that BldC has evolved to play different roles in morphological differentiation between the two genera of filamentous actinomycetes.

An integrative-omics analysis of an industrial clavulanic acid-overproducing Streptomyces clavuligerus

Clavulanic acid (CA) is a clinically important secondary metabolite used to treat infectious diseases. We aimed to decipher complex regulatory mechanisms acting in CA biosynthesis by analyzing transcriptome- and proteome-wide alterations in an industrial CA overproducer Streptomyces clavuligerus strain, namely DEPA and its wild-type counterpart NRRL3585. A total of 924 differentially expressed genes (DEGs) and 271 differentially produced proteins (DPPs) were obtained by RNA-seq and nanoLC-MS/MS analyses, respectively. In particular, CA biosynthetic genes, namely, car (cad), cas2, oat2, pah, bls, ceas2, orf12, and claR, a cluster situated regulatory (CSR) gene, were significantly upregulated as shown by RNA-seq. Enzymes of clavam biosynthesis were downregulated considerably in the DEPA strain, while the genes involved in the arginine biosynthesis, one of the precursors of CA pathway, were overexpressed. However, the biosynthesis of the other CA precursor, glyceraldehyde-3-phosphate (G3P), was not affected. CA overproduction in the DEPA strain was correlated with BldD, BldG, BldM, and BldN (AdsA) overrepresentation. In addition, TetR, WhiB, and Xre family transcriptional regulators were shown to be significantly overrepresented. Several uncharacterized/unknown proteins differentially expressed in the DEPA strain await further studies for functional characterization. Correlation analysis indicated an acceptable degree of consistency between the transcriptome and proteome data. The study represents the first integrative-omics analysis in a CA overproducer S. clavuligerus strain, providing insights into the critical control points and potential rational engineering targets for a purposeful increase of CA yields in strain improvement. KEY POINTS: ∙ Transcriptome and proteome-wide alterations in industrial CA overproducer strain DEPA ∙ An acceptable degree of consistency between the transcriptome and proteome data ∙ New targets to be exploited for rational engineering.

Genome-Wide Identification of the LexA-Mediated DNA Damage Response in Streptomyces venezuelae

DNA damage triggers a widely conserved stress response in bacteria called the SOS response, which involves two key regulators, the activator RecA and the transcriptional repressor LexA. Despite the wide conservation of the SOS response, the number of genes controlled by LexA varies considerably between different organisms. The filamentous soil-dwelling bacteria of the genus Streptomyces contain LexA and RecA homologs, but their roles in Streptomyces have not been systematically studied. Here, we demonstrate that RecA and LexA are required for the survival of Streptomyces venezuelae during DNA-damaging conditions and for normal development during unperturbed growth. Monitoring the activity of a fluorescent recA promoter fusion and LexA protein levels revealed that the activation of the SOS response is delayed in S. venezuelae. By combining global transcriptional profiling and chromatin immunoprecipitation sequencing (ChIP-seq) analysis, we determined the LexA regulon and defined the core set of DNA damage repair genes that are expressed in response to treatment with the DNA-alkylating agent mitomycin C. Our results show that DNA damage-induced degradation of LexA results in the differential regulation of LexA target genes. Using surface plasmon resonance, we further confirmed the LexA DNA binding motif (SOS box) and demonstrated that LexA displays tight but distinct binding affinities to its target promoters, indicating a graded response to DNA damage.

IMPORTANCE The transcriptional regulator LexA functions as a repressor of the bacterial SOS response, which is induced under DNA-damaging conditions. This results in the expression of genes important for survival and adaptation. Here, we report the regulatory network controlled by LexA in the filamentous antibiotic-producing Streptomyces bacteria and establish the existence of the SOS response in Streptomyces. Collectively, our work reveals significant insights into the DNA damage response in Streptomyces that will promote further studies to understand how these important bacteria adapt to their environment.

ActinoBase: tools and protocols for researchers working on Streptomyces and other filamentous actinobacteria

Actinobacteria is an ancient phylum of Gram-positive bacteria with a characteristic high GC content to their DNA. The ActinoBase Wiki is focused on the filamentous actinobacteria, such as Streptomyces species, and the techniques and growth conditions used to study them. These organisms are studied because of their complex developmental life cycles and diverse specialised metabolism which produces many of the antibiotics currently used in the clinic. ActinoBase is a community effort that provides valuable and freely accessible resources, including protocols and practical information about filamentous actinobacteria. It is aimed at enabling knowledge exchange between members of the international research community working with these fascinating bacteria. ActinoBase is an anchor platform that underpins worldwide efforts to understand the ecology, biology and metabolic potential of these organisms. There are two key differences that set ActinoBase apart from other Wiki-based platforms: [1] ActinoBase is specifically aimed at researchers working on filamentous actinobacteria and is tailored to help users overcome challenges working with these bacteria and [2] it provides a freely accessible resource with global networking opportunities for researchers with a broad range of experience in this field.

DNA damage-induced block of sporulation in Streptomyces venezuelae involves downregulation of ssgB

DNA damage often causes an arrest of the cell cycle that provides time for genome integrity to be restored. In bacteria, the classical SOS DNA damage response leads to an inhibition of cell division resulting in temporarily filamentous growth. This raises the question as to whether such a response mechanism might similarly function in naturally filamentous bacteria such as Streptomyces. Streptomyces exhibit two functionally distinct forms of cell division: cross-wall formation in vegetative hyphae and sporulation septation in aerial hyphae. Here, we show that the genotoxic agent mitomycin C confers a block in sporulation septation in Streptomyces venezuelae in a mechanism that involves, at least in part, the downregulation of ssgB. Notably, this DNA damage response does not appear to block cross-wall formation and may be independent of canonical SOS and developmental regulators. We also show that the mitomycin C-induced block in sporulation can be partially bypassed by the constitutive expression of ssgB, though this appears to be largely limited to mitomycin C treatment and the resultant spore-like cells have reduced viability.

AdpA, a developmental regulator, promotes ε-poly-l-lysine biosynthesis in Streptomyces albulus

Background

AdpA is a global regulator of morphological differentiation and secondary metabolism in Streptomyces, but the regulatory roles of the Streptomyces AdpA family on the biosynthesis of the natural product ε-poly-l-lysine (ε-PL) remain unidentified, and few studies have focused on increasing the production of ε-PL by manipulating transcription factors in Streptomyces.

Results

In this study, we revealed the regulatory roles of different AdpA homologs in ε-PL biosynthesis and morphological differentiation and effectively promoted ε-PL production and sporulation in Streptomycesalbulus NK660 by heterologously expressing adpA from S.neyagawaensis NRRLB-3092 (adpA_Sn). First, we identified a novel AdpA homolog named AdpA_Sa in S.albulus NK660 and characterized its function as an activator of ε-PL biosynthesis and morphological differentiation. Subsequently, four heterologous AdpA homologs were selected to investigate their phylogenetic relationships and regulatory roles in S.albulus, and AdpA_Sn was demonstrated to have the strongest ability to promote both ε-PL production and sporulation among these five AdpA proteins. The ε-PL yield of S.albulus heterologously expressing adpA_Sn was approximately 3.6-fold higher than that of the control strain. Finally, we clarified the mechanism of AdpA_Sn in enhancing ε-PL biosynthesis and its effect on ε-PL polymerization degree using real-time quantitative PCR, microscale thermophoresis and MALDI-TOF–MS. AdpA_Sn was purified, and its seven direct targets, zwf, tal, pyk2, pta, ack, pepc and a transketolase gene (DC74_2409), were identified, suggesting that AdpA_Sn may cause the redistribution of metabolic flux in central metabolism pathways, which subsequently provides more carbon skeletons and ATP for ε-PL biosynthesis in S.albulus.

Conclusions

Here, we characterized the positive regulatory roles of Streptomyces AdpA homologs in ε-PL biosynthesis and their effects on morphological differentiation and reported for the first time that AdpA_Sn promotes ε-PL biosynthesis by affecting the transcription of its target genes in central metabolism pathways. These findings supply valuable insights into the regulatory roles of the Streptomyces AdpA family on ε-PL biosynthesis and morphological differentiation and suggest that AdpA_Sn may be an effective global regulator for enhanced production of ε-PL and other valuable secondary metabolites in Streptomyces.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01785-6.

Biological Functions and Applications of Virus-Related Bacterial Nanoparticles: A Review

Accumulating evidence suggests that microorganisms produce various nanoparticles that exhibit a variety of biological functions. The structure of these bacterial nanoparticles ranges from membrane vesicles composed of membrane lipids to multicomponent proteinaceous machines. Of bacterial nanoparticles, bacterial phage tail-like nanoparticles, associated with virus-related genes, are found in bacteria from various environments and have diverse functions. Extracellular contractile injection systems (eCISs), a type of bacterial phage tail-like nanostructure, have diverse biological functions that mediate the interactions between the producer bacteria and target eukaryote. Known gram-negative bacterial eCISs can act as protein translocation systems and inject effector proteins that modulate eukaryotic cellular processes by attaching to the target cells. Further investigation of the functions of eCISs will facilitate the application of these nanomachines as nano-sized syringes in the field of nanomedicine and vaccine development. This review summarises the recent progress in elucidating the structures and biological functions of nanoparticles that resemble the tail components of phages that infect bacteria and discusses directions for future research to improve the clinical applicability of virus-related bacterial nanoparticles.

Interplay between non-coding RNA transcription, stringent phenotype and antibiotic production in Streptomyces

While in recent years the key role of non-coding RNAs (ncRNAs) in regulation of gene expression has become increasingly evident, their interaction with the global regulatory circuits is still obscure. Here we analyzed the structure and organization of the transcriptome of Streptomyces ambofaciens, the producer of spiramycin. We identified ncRNAs including 45 small-RNAs (sRNAs) and 119 antisense-RNAs (asRNAs I) that appear transcribed from dedicated promoters. Some sRNAs and asRNAs are unprecedented in Streptomyces, and were predicted to target mRNAs encoding proteins involved in transcription, translation, ribosomal structure and biogenesis, and regulation of morphological and biochemical differentiation. We then compared ncRNA expression in three strains: i.) the wild type strain; ii.) an isogenic pirA-defective mutant with central carbon metabolism imbalance, "relaxed" phenotype, and repression of antibiotic production; iii.) a pirA-derivative strain harboring a "stringent" RNA polymerase that suppresses pirA-associated phenotypes. Data indicated that expression of most ncRNAs was correlated to the stringent/relaxed phenotype suggesting novel effector mechanisms of the stringent response.

BldD-based bimolecular fluorescence complementation for in vivo detection of the second messenger cyclic di-GMP

The widespread bacterial second messenger bis-(3'-5')-cyclic diguanosine monophosphate (c-di-GMP) is an important regulator of biofilm formation, virulence and cell differentiation. C-di-GMP-specific biosensors that allow detection and visualization of c-di-GMP levels in living cells are key to our understanding of how c-di-GMP fluctuations drive cellular responses. Here, we describe a novel c-di-GMP biosensor, CensYBL, that is based on c-di-GMP-induced dimerization of the effector protein BldD from Streptomyces resulting in bimolecular fluorescence complementation of split-YPet fusion proteins. As a proof-of-principle, we demonstrate that CensYBL is functional in detecting fluctuations in intracellular c-di-GMP levels in the Gram-negative model bacteria Escherichia coli and Salmonella enterica serovar Typhimurium. Using deletion mutants of c-di-GMP diguanylate cyclases and phosphodiesterases, we show that c-di-GMP dependent dimerization of CBldD-YPet results in fluorescence complementation reflecting intracellular c-di-GMP levels. Overall, we demonstrate that the CensYBL biosensor is a user-friendly and versatile tool that allows to investigate c-di-GMP variations using single-cell and population-wide experimental set-ups.

AdpA Positively Regulates Morphological Differentiation and Chloramphenicol Biosynthesis in Streptomyces venezuelae

In members of genus Streptomyces, AdpA is a master transcriptional regulator that controls the expression of hundreds of genes involved in morphological differentiation, secondary metabolite biosynthesis, chromosome replication, etc. However, the function of AdpASv, an AdpA ortholog of Streptomyces venezuelae, is unknown. This bacterial species is a natural producer of chloramphenicol and has recently become a model organism for studies on Streptomyces. Here, we demonstrate that AdpASv is essential for differentiation and antibiotic biosynthesis in S. venezuelae and provide evidence suggesting that AdpASv positively regulates its own gene expression. We speculate that the different modes of AdpA-dependent transcriptional autoregulation observed in S. venezuelae and other Streptomyces species reflect the arrangement of AdpA binding sites in relation to the transcription start site. Lastly, we present preliminary data suggesting that AdpA may undergo a proteolytic processing and we speculate that this may potentially constitute a novel regulatory mechanism controlling cellular abundance of AdpA in Streptomyces. IMPORTANCEStreptomyces are well-known producers of valuable secondary metabolites which include a large variety of antibiotics and important model organisms for developmental studies in multicellular bacteria. The conserved transcriptional regulator AdpA of Streptomyces exerts a pleiotropic effect on cellular processes, including the morphological differentiation and biosynthesis of secondary metabolites. Despite extensive studies, the function of AdpA in these processes remains elusive. This work provides insights into the role of a yet unstudied AdpA ortholog of Streptomyces venezuelae, now considered a novel model organism. We found that AdpA plays essential role in morphological differentiation and biosynthesis of chloramphenicol, a broad-spectrum antibiotic. We also propose that AdpA may undergo a proteolytic processing that presumably constitutes a novel mechanism regulating cellular abundance of this master regulator.

Re-classification of Streptomyces venezuelae strains and mining secondary metabolite biosynthetic gene clusters

Streptomyces species have attracted considerable interest as a reservoir of medically important secondary metabolites, which are even diverse and different between strains. Here, we reassess ten Streptomyces venezuelae strains by presenting the highly resolved classification, using 16S rRNA sequencing, MALDI-TOF MS protein profiling, and whole-genome sequencing. The results revealed that seven of the ten strains were misclassified as S. venezuelae species. Secondary metabolite biosynthetic gene cluster (smBGC) mining and targeted LC-MS/MS based metabolite screening of S. venezuelae and misclassified strains identified in total 59 secondary metabolites production. In addition, a comparison of pyrrolamide-type antibiotic BGCs of four misclassified strains, followed by functional genomics, revealed that athv28 is critical in the synthesis of the anthelvencin precursor, 5-amino-3,4-dihydro-2H-pyrrole-2-carboxylate (ADPC). Our findings illustrate the importance of the accurate classification and better utilization of misclassified Streptomyces strains to discover smBGCs and their secondary metabolite products.

Global Chromosome Topology and the Two-Component Systems in Concerted Manner Regulate Transcription in Streptomyces

Bacterial gene expression is controlled at multiple levels, with chromosome supercoiling being one of the most global regulators. Global DNA supercoiling is maintained by the orchestrated action of topoisomerases. In Streptomyces, mycelial soil bacteria with a complex life cycle, topoisomerase I depletion led to elevated chromosome supercoiling, changed expression of a significant fraction of genes, delayed growth, and blocked sporulation. To identify supercoiling-induced sporulation regulators, we searched for Streptomyces coelicolor transposon mutants that were able to restore sporulation despite high chromosome supercoiling. We established that transposon insertion in genes encoding a novel two-component system named SatKR reversed the sporulation blockage resulting from topoisomerase I depletion. Transposition in satKR abolished the transcriptional induction of the genes within the so-called supercoiling-hypersensitive cluster (SHC). Moreover, we found that activated SatR also induced the same set of SHC genes under normal supercoiling conditions. We determined that the expression of genes in this region impacted S. coelicolor growth and sporulation. Interestingly, among the associated products is another two-component system (SitKR), indicating the potential for cascading regulatory effects driven by the SatKR and SitKR two-component systems. Thus, we demonstrated the concerted activity of chromosome supercoiling and a hierarchical two-component signaling system that impacts gene activity governing Streptomyces growth and sporulation.

IMPORTANCEStreptomyces microbes, soil bacteria with complex life cycle, are the producers of a broad range of biologically active compounds (e.g., antibiotics). Streptomyces bacteria respond to various environmental signals using a complex transcriptional regulation mechanism. Understanding regulation of their gene expression is crucial for Streptomyces application as industrial organisms. Here, on the basis of the results of extensive transcriptomics analyses, we describe the concerted gene regulation by global DNA supercoiling and novel two-component system. Our data indicate that regulated genes encode growth and sporulation regulators. Thus, we demonstrate that Streptomyces bacteria link the global regulatory strategies to adjust life cycle to unfavorable conditions.

Cell density, alignment, and orientation correlate with C-signal-dependent gene expression during Myxococcus xanthus development

Starving Myxococcus xanthus bacteria use short-range C-signaling to coordinate their movements and construct multicellular mounds, which mature into fruiting bodies as rods differentiate into spherical spores. Differentiation requires efficient C-signaling to drive the expression of developmental genes, but how the arrangement of cells within nascent fruiting bodies (NFBs) affects C-signaling is not fully understood. Here, we used confocal microscopy and cell segmentation to visualize and quantify the arrangement, morphology, and gene expression of cells near the bottom of NFBs at much higher resolution than previously achieved. We discovered that "transitioning cells" (TCs), intermediate in morphology between rods and spores, comprised 10 to 15% of the total population. Spores appeared midway between the center and the edge of NFBs early in their development and near the center as maturation progressed. The developmental pattern, as well as C-signal-dependent gene expression in TCs and spores, were correlated with cell density, the alignment of neighboring rods, and the tangential orientation of rods early in the development of NFBs. These dynamic radial patterns support a model in which the arrangement of cells within the NFBs affects C-signaling efficiency to regulate precisely the expression of developmental genes and cellular differentiation in space and time. Developmental patterns in other bacterial biofilms may likewise rely on short-range signaling to communicate multiple aspects of cellular arrangement, analogous to juxtacrine and paracrine signaling during animal development.

An Intracellular Sensing and Signal Transduction System That Regulates the Metabolism of Polycyclic Aromatic Hydrocarbons in Bacteria

Many bacteria utilize polycyclic aromatic hydrocarbon (PAH) as carbon and energy sources for growth. These bacteria play an important role in the amelioration of PAH pollution in various environments. However, it is unclear how bacteria sense PAHs and how PAH degradation pathways are regulated via signal transduction. Here, we investigated these mechanisms in Cycloclasticus, a ubiquitous PAH-degrading bacterium in marine environments. We identified the key genes involved in intracellular PAH sensing, signal transduction, and the differential regulation of degradation pathways for each PAH examined. Our results showed that PAHs bind specifically to a diguanylate cyclase PdgC, leading to the generation of cyclic dimeric GMP (c-di-GMP), which subsequently binds to two CRP/FNR family regulators, DPR-1 and DPR-2. c-di-GMP activates the transcription of DPR-1 and DPR-2 to positively regulate degradation pathways specific to pyrene and phenanthrene/naphthalene, respectively. This is the first report of an intracellular signal transduction pathway associated with PAH degradation in bacteria. Our results improve our understanding of the intracellular responses to PAHs. The existence of the identified genes in other bacteria indicates that the strategy described here is widely used by other PAH-degrading bacteria. IMPORTANCE Polycyclic aromatic hydrocarbons (PAHs) are widely distributed and have been found indoors, in the atmosphere, in terrestrial soils, in marine waters and sediments, and even in outer space. Bacteria degrade PAHs via degradation pathways. PAH signal sensing and transduction, as well as the regulation of PAH degradation pathways, are crucial for bacterial PAH biodegradation. However, prior to this study, these processes were poorly known. This study employed multiple molecular approaches to better understand the regulatory networks controlling PAH metabolism in bacteria. This report illustrates, for the first time, PAH-specific intracellular sensing, signal transduction, and metabolic regulatory pathways. Our results will help to increase our understanding of the hydrocarbon-metabolism regulatory network as well as the regulatory intricacies that control microbial biodegradation of organic matter. These key data should be considered to improve the rational design and efficiency of recombinant biodegradable, bacterial biosensors, and biocatalysts in modern green chemistry.

Phase-variable expression of pdcB, a phosphodiesterase, influences sporulation in Clostridioides difficile

Clostridioides difficile is the causative agent of antibiotic-associated diarrhea and is the leading cause of nosocomial infection in developed countries. An increasing number of C. difficile infections are attributed to epidemic strains that produce more toxins and spores. C. difficile spores are the major factor for the transmission and persistence of the organism. Previous studies have identified global regulators that influence sporulation in C. difficile. This study discovers that PdcB, a phosphodiesterase, enhances sporulation in C. difficile strain UK1. Through genetic and biochemical assays, we show that phase-variable expression of pdcB results in hypo- and hyper-sporulation phenotypes. In the "ON" orientation, the identified promotor is in the right orientation to drive the expression of pdcB. Production of the PdcB phosphodiesterase reduces the intracellular cyclic-di-GMP (c-di-GMP) concentration, resulting in a hyper-sporulation phenotype. Loss of PdcB due to the pdcB promoter being in the OFF orientation or mutation of pdcB results in increased c-di-GMP levels and a hypo-sporulation phenotype. Additionally, we demonstrate that CodY binds to the upstream region of pdcB. DNA inversion reorients the CodY binding site so that in the OFF orientation, CodY binds a site that is upstream of the pdcB promoter and can further repress gene expression.

Integrating perspectives in actinomycete research: an ActinoBase review of 2020-21

Last year ActinoBase, a Wiki-style initiative supported by the UK Microbiology Society, published a review highlighting the research of particular interest to the actinomycete community. Here, we present the second ActinoBase review showcasing selected reports published in 2020 and early 2021, integrating perspectives in the actinomycete field. Actinomycetes are well-known for their unsurpassed ability to produce specialised metabolites, of which many are used as therapeutic agents with antibacterial, antifungal, or immunosuppressive activities. Much research is carried out to understand the purpose of these metabolites in the environment, either within communities or in host interactions. Moreover, many efforts have been placed in developing computational tools to handle big data, simplify experimental design, and find new biosynthetic gene cluster prioritisation strategies. Alongside, synthetic biology has provided advances in tools to elucidate the biosynthesis of these metabolites. Additionally, there are still mysteries to be uncovered in understanding the fundamentals of filamentous actinomycetes' developmental cycle and regulation of their metabolism. This review focuses on research using integrative methodologies and approaches to understand the bigger picture of actinomycete biology, covering four research areas: i) technology and methodology; ii) specialised metabolites; iii) development and regulation; and iv) ecology and host interactions.

Constructing Smartphone-Controlled Optogenetic Switches in Mammalian Cells

With the increasing indispensable role of smartphones in our daily lives, the mobile health care system coupled with embedded physical sensors and modern communication technologies make it an attractive technology for enabling the remote monitoring of an individual's health. Using a multidisciplinary design principle coupled with smart electronics, software, and optogenetics, the investigators constructed smartphone-controlled optogenetic switches to enable the ultraremote-control transgene expression. A custom-designed SmartController system was programmed to process wireless signals from smartphones, enabling the regulation of therapeutic outputs production by optically engineered cells via a far-red light (FRL)-responsive optogenetic interface. In the present study, the investigators describe the details of the protocols for constructing smartphone-controlled optogenetic switches, including the rational design of an FRL-triggered transgene expression circuit, the procedure for cell culture and transfection, the implementation of the smartphone-controlled far-red light-emitting diode (LED) module, and the reporter detection assay.

Interplay between Non-Coding RNA Transcription, Stringent/Relaxed Phenotype and Antibiotic Production in Streptomyces ambofaciens

While in recent years the key role of non-coding RNAs (ncRNAs) in the regulation of gene expression has become increasingly evident, their interaction with the global regulatory circuits is still obscure. Here we analyzed the structure and organization of the transcriptome of Streptomyces ambofaciens, the producer of spiramycin. We identified ncRNAs including 45 small-RNAs (sRNAs) and 119 antisense-RNAs (asRNAs I) that appear transcribed from dedicated promoters. Some sRNAs and asRNAs are unprecedented in Streptomyces and were predicted to target mRNAs encoding proteins involved in transcription, translation, ribosomal structure and biogenesis, and regulation of morphological and biochemical differentiation. We then compared ncRNA expression in three strains: (i) the wild-type strain; (ii) an isogenic pirA-defective mutant with central carbon metabolism imbalance, “relaxed” phenotype, and repression of antibiotic production; and (iii) a pirA-derivative strain harboring a “stringent” RNA polymerase that suppresses pirA-associated phenotypes. Data indicated that the expression of most ncRNAs was correlated to the stringent/relaxed phenotype suggesting novel effector mechanisms of the stringent response.

Evolution of a σ-(c-di-GMP)-anti-σ switch

Diverse bacterial lifestyle transitions are controlled by the nucleotide second messenger c-di-GMP, including virulence, motility, and biofilm formation. To control such fundamentally distinct processes, the set of genes under c-di-GMP control must have gone through several shifts during bacterial evolution. Here we show that the same σ–(c-di-GMP)–anti-σ switch has been co-opted during evolution to regulate distinct biological functions in unicellular and filamentous bacteria, controlling type IV pilus production in the genus Rubrobacter and the differentiation of reproductive hyphae into spores in Streptomyces. Moreover, we show that the anti-σ likely originated as a homodimer and evolved to become a monomer through an intragenic duplication event. This study thus describes the structural and functional evolution of a c-di-GMP regulatory switch.

Filamentous actinobacteria of the genus Streptomyces have a complex lifecycle involving the differentiation of reproductive aerial hyphae into spores. We recently showed c-di-GMP controls this transition by arming a unique anti-σ, RsiG, to bind the sporulation-specific σ, WhiG. The Streptomyces venezuelae RsiG–(c-di-GMP)₂–WhiG structure revealed that a monomeric RsiG binds c-di-GMP via two E(X)₃S(X)₂R(X)₃Q(X)₃D repeat motifs, one on each helix of an antiparallel coiled-coil. Here we show that RsiG homologs are found scattered throughout the Actinobacteria. Strikingly, RsiGs from unicellular bacteria descending from the most basal branch of the Actinobacteria are small proteins containing only one c-di-GMP binding motif, yet still bind their WhiG partners. Our structure of a Rubrobacter radiotolerans (RsiG)₂–(c-di-GMP)₂–WhiG complex revealed that these single-motif RsiGs are able to form an antiparallel coiled-coil through homodimerization, thereby allowing them to bind c-di-GMP similar to the monomeric twin-motif RsiGs. Further data show that in the unicellular actinobacterium R. radiotolerans, the (RsiG)₂–(c-di-GMP)₂–WhiG regulatory switch controls type IV pilus expression. Phylogenetic analysis indicates the single-motif RsiGs likely represent the ancestral state and an internal gene-duplication event gave rise to the twin-motif RsiGs inherited elsewhere in the Actinobacteria. Thus, these studies show how the anti-σ RsiG has evolved through an intragenic duplication event from a small protein carrying a single c-di-GMP binding motif, which functions as a homodimer, to a larger protein carrying two c-di-GMP binding motifs, which functions as a monomer. Consistent with this, our structures reveal potential selective advantages of the monomeric twin-motif anti-σ factors.

Streptomyces venezuelae NRRL B-65442: genome sequence of a model strain used to study morphological differentiation in filamentous actinobacteria

For over a decade, Streptomyces venezuelae has been used to study the molecular mechanisms that control morphological development in streptomycetes and it is now a well-established model strain. Its rapid growth and ability to sporulate in a near-synchronised manner in liquid culture, unusual among streptomycetes, greatly facilitates the application of modern molecular techniques such as ChIP-seq and RNA-seq, as well as fluorescence time-lapse imaging of the complete Streptomyces life cycle. Here we describe a high-quality genome sequence of our isolate of the strain (NRRL B-65442) consisting of an 8.2 Mb chromosome and a 158 kb plasmid, pSVJI1, which had not been reported previously. Surprisingly, while NRRL B-65442 yields green spores on MYM agar, the ATCC type strain 10712 (from which NRRL B-65442 was derived) produces grey spores. While comparison of the genome sequences of the two isolates revealed almost total identity, it did reveal a single nucleotide substitution in a gene, vnz_33525, likely to be involved in spore pigment biosynthesis. Replacement of the vnz_33525 allele of ATCC 10712 with that of NRRL B-65442 resulted in green spores, explaining the discrepancy in spore pigmentation. We also applied CRISPR-Cas9 to delete the essential parB of pSVJI1 to cure the plasmid from the strain without obvious phenotypic consequences.

Genetic Network Architecture and Environmental Cues Drive Spatial Organization of Phenotypic Division of Labor in Streptomyces coelicolor

A number of bacteria are known to differentiate into cells with distinct phenotypic traits during processes such as biofilm formation or the development of reproductive structures. These cell types, by virtue of their specialized functions, embody a division of labor. However, how bacteria build spatial patterns of differentiated cells is not well understood. Here, we examine the factors that drive spatial patterns in divisions of labor in colonies of Streptomyces coelicolor, a multicellular bacterium capable of synthesizing an array of antibiotics and forming complex reproductive structures (e.g., aerial hyphae and spores). Using fluorescent reporters, we demonstrate that the pathways for antibiotic biosynthesis and aerial hypha formation are activated in distinct waves of gene expression that radiate outwards in S. coelicolor colonies. We also show that the spatiotemporal separation of these cell types depends on a key activator in the developmental pathway, AdpA. Importantly, when we manipulated local gradients by growing competing microbes nearby, or through physical disruption, expression in these pathways could be decoupled and/or disordered, respectively. Finally, the normal spatial organization of these cell types was partially restored with the addition of a siderophore, a public good made by these organisms, to the growth medium. Together, these results indicate that spatial divisions of labor in S. coelicolor colonies are determined by a combination of physiological gradients and regulatory network architecture, key factors that also drive patterns of cellular differentiation in multicellular eukaryotic organisms.

A conserved cell division protein directly regulates FtsZ dynamics in filamentous and unicellular actinobacteria

Bacterial cell division is driven by the polymerization of the GTPase FtsZ into a contractile structure, the so-called Z-ring. This essential process involves proteins that modulate FtsZ dynamics and hence the overall Z-ring architecture. Actinobacteria like Streptomyces and Mycobacterium lack known key FtsZ-regulators. Here we report the identification of SepH, a conserved actinobacterial protein that directly regulates FtsZ dynamics. We show that SepH is crucially involved in cell division in Streptomyces venezuelae and that it binds FtsZ via a conserved helix-turn-helix motif, stimulating the assembly of FtsZ protofilaments. Comparative in vitro studies using the SepH homolog from Mycobacterium smegmatis further reveal that SepH can also bundle FtsZ protofilaments, indicating an additional Z-ring stabilizing function in vivo. We propose that SepH plays a crucial role at the onset of cytokinesis in actinobacteria by promoting the assembly of FtsZ filaments into division-competent Z-rings that can go on to mediate septum synthesis.

Influence of core divisome proteins on cell division in Streptomyces venezuelae ATCC 10712

The sporulating, filamentous soil bacterium Streptomyces venezuelae ATCC 10712 differentiates under submerged and surface growth conditions. In order to lay a solid foundation for the study of development-associated division for this organism, a congenic set of mutants was isolated, individually deleted for a gene encoding either a cytoplasmic (i.e. ftsZ) or core inner membrane (i.e. divIC, ftsL, ftsI, ftsQ, ftsW) component of the divisome. While ftsZ mutants are completely blocked for division, single mutants in the other core divisome genes resulted in partial, yet similar, blocks in sporulation septum formation. Double and triple mutants for core divisome membrane components displayed phenotypes that were similar to those of the single mutants, demonstrating that the phenotypes were not synergistic. Division in this organism is still partially functional without multiple core divisome proteins, suggesting that perhaps other unknown lineage-specific proteins perform redundant functions. In addition, by isolating an ftsZ2p mutant with an altered -10 region, the conserved developmentally controlled promoter was also shown to be required for sporulation-associated division. Finally, microscopic observation of FtsZ-YFP dynamics in the different mutant backgrounds led to the conclusion that the initial assembly of regular Z rings does not per se require the tested divisome membrane proteins, but the stability of Z rings is dependent on the divisome membrane components tested. The observation is consistent with the interpretation that Z ring instability likely results from and further contributes to the observed defects in sporulation septation in mutants lacking core divisome proteins.

The novel ECF56 SigG1-RsfG system modulates morphological differentiation and metal-ion homeostasis in Streptomyces tsukubaensis

Extracytoplasmic function (ECF) sigma factors are key transcriptional regulators that prokaryotes have evolved to respond to environmental challenges. Streptomyces tsukubaensis harbours 42 ECFs to reprogram stress-responsive gene expression. Among them, SigG1 features a minimal conserved ECF σ2-σ4 architecture and an additional C-terminal extension that encodes a SnoaL_2 domain, which is characteristic for ECF σ factors of group ECF56. Although proteins with such domain organisation are widely found among Actinobacteria, the functional role of ECFs with a fused SnoaL_2 domain remains unknown. Our results show that in addition to predicted self-regulatory intramolecular amino acid interactions between the SnoaL_2 domain and the ECF core, SigG1 activity is controlled by the cognate anti-sigma protein RsfG, encoded by a co-transcribed sigG1-neighbouring gene. Characterisation of ∆sigG1 and ∆rsfG strains combined with RNA-seq and ChIP-seq experiments, suggests the involvement of SigG1 in the morphological differentiation programme of S. tsukubaensis. SigG1 regulates the expression of alanine dehydrogenase, ald and the WhiB-like regulator, wblC required for differentiation, in addition to iron and copper trafficking systems. Overall, our work establishes a model in which the activity of a σ factor of group ECF56, regulates morphogenesis and metal-ions homeostasis during development to ensure the timely progression of multicellular differentiation.

Sulfane sulfur-activated actinorhodin production and sporulation is maintained by a natural gene circuit in Streptomyces coelicolor

Sulfane sulfur, including polysulfide and persulfide, is a newly identified cellular component present in microorganisms; however, its physiological functions are unclear. Streptomyces coelicolor M145 is a model strain of actinomycetes, which produces several polyketides, including actinorhodin. Herein, we found that both exogenously added and endogenously generated sulfane sulfur increased the actinorhodin production and accelerated spore formation of S. coelicolor M145. This bacterial species carries a natural gene circuit containing four genes that encode a CsoR-like transcription factor (ScCsoR), persulfide dioxygenase (ScPDO), rhodanese and a sulfite transporter, which were shown to be responsible for sensing and removal of excessive sulfane sulfur. ScCsoR was observed to bind to the promoters of the four genes, thus repressing their transcription. Sulfane sulfur modified Cys37 of ScCsoR, and the modified ScCSoR did not bind to the promoters, thereby activating the transcription of ScPDO. The deletion of ScCsoR decreased cellular sulfane sulfur, while the deletion of ScPDO increased its levels. The increased sulfane sulfur promoted actinorhodin production and sporulation. This study unveiled a natural gene circuit for maintaining sulfane sulfur homeostasis in bacteria. Further, we identified the trigger effect of sulfane sulfur on actinorhodin production, presenting a new approach for activating polyketide gene clusters in actinomycetes.

Specialized and shared functions of diguanylate cyclases and phosphodiesterases in Streptomyces development

The second messenger bis-3,5-cyclic di-guanosine monophosphate (c-di-GMP) determines when Streptomyces initiate sporulation. c-di-GMP signals are integrated into the genetic differentiation network by the regulator BldD and the sigma factor σ^WhiG . However, functions of the development-specific diguanylate cyclases (DGCs) CdgB and CdgC, and the c-di-GMP phosphodiesterases (PDEs) RmdA and RmdB, are poorly understood. Here, we provide biochemical evidence that the GGDEF-EAL domain protein RmdB from S. venezuelae is a monofunctional PDE that hydrolyzes c-di-GMP to 5'pGpG. Despite having an equivalent GGDEF-EAL domain arrangement, RmdA cleaves c-di-GMP to GMP and exhibits residual DGC activity. We show that an intact EAL motif is crucial for the in vivo function of both enzymes since strains expressing protein variants with an AAA motif instead of EAL are delayed in development, similar to null mutants. Transcriptome analysis of ∆cdgB, ∆cdgC, ∆rmdA, and ∆rmdB strains revealed that the c-di-GMP specified by these enzymes has a global regulatory role, with about 20% of all S. venezuelae genes being differentially expressed in the cdgC mutant. Our data suggest that the major c-di-GMP-controlled targets determining the timing and mode of sporulation are genes involved in cell division and the production of the hydrophobic sheath that covers Streptomyces aerial hyphae and spores.

Physiological and Transcriptional Responses of Streptomyces albulus to Acid Stress in the Biosynthesis of ε-Poly-L-lysine

Streptomyces albulus has commercially been used for the production of ε-poly-L-lysine (ε-PL), a natural food preservative, where acid stress is inevitably encountered in the biosynthesis process. To elucidate the acid tolerance response (ATR), a comparative physiology and transcriptomic analysis of S. albulus M-Z18 at different environmental pH (5.0, 4.0, and 3.0) was carried out. In response to acid stress, cell envelope regulated the membrane fatty acid composition and chain length to reduce damage. Moreover, intracellular pH homeostasis was maintained by increasing H⁺-ATPase activity and intracellular ATP and amino acid (mainly arginine, glutamate, aspartate and lysine) concentrations. Transcriptional analysis based on RNA-sequencing indicated that acid stress aroused global changes and the differentially expressed genes involved in transcriptional regulation, stress-response protein, transporter, cell envelope, secondary metabolite biosynthesis, DNA and RNA metabolism and ribosome subunit. Consequently, the ATR of S. albulus was preliminarily proposed. Notably, it is indicated that the biosynthesis of ε-PL is also a response mechanism for S. albulus to combat acid stress. These results provide new insights into the ATR of S. albulus and will contribute to the production of ε-PL via adaptive evolution or metabolic engineering.

Comparative Genomics Determines Strain-Dependent Secondary Metabolite Production in Streptomyces venezuelae Strains

Streptomyces venezuelae is well known to produce various secondary metabolites, including chloramphenicol, jadomycin, and pikromycin. Although many strains have been classified as S. venezuelae species, only a limited number of strains have been explored extensively for their genomic contents. Moreover, genomic differences and diversity in secondary metabolite production between the strains have never been compared. Here, we report complete genome sequences of three S. venezuelae strains (ATCC 10712, ATCC 10595, and ATCC 21113) harboring chloramphenicol and jadomycin biosynthetic gene clusters (BGC). With these high-quality genome sequences, we revealed that the three strains share more than 85% of total genes and most of the secondary metabolite biosynthetic gene clusters (smBGC). Despite such conservation, the strains produced different amounts of chloramphenicol and jadomycin, indicating differential regulation of secondary metabolite production at the strain level. Interestingly, antagonistic production of chloramphenicol and jadomycin was observed in these strains. Through comparison of the chloramphenicol and jadomycin BGCs among the three strains, we found sequence variations in many genes, the non-coding RNA coding regions, and binding sites of regulators, which affect the production of the secondary metabolites. We anticipate that these genome sequences of closely related strains would serve as useful resources for understanding the complex secondary metabolism and for designing an optimal production process using Streptomyces strains.